Although various colorless transparent materials have been investigated depending upon various uses such as functional optical films, disc substrates, etc., the functions and performance required for the materials themselves have become more precise and more high-level accompanying with a rapid change of information equipment to be small-sized and light-weight and a development of display elements to be high-fineness.
Particularly, in film fields, films using polycarbonate-group, cyclopolyolefin-group and cellulose-group resins have been used as display members such as polarizing plates or retardation plates and as disc protection films.
Thermoplastic resins having a high transparency such as a polycarbonate are employed broadly as an optical use, and they are considered to be used as optical films such as retardation films and substrates for discs. Especially, a retardation film is one of the important structural members deciding a contrast of a reflection type color liquid crystal display. Although polycarbonates used at the present time are described in, for example, JP-A-4-204503 and JP-A-9-304619 do not have a satisfactory wavelength dispersion property. In order to make a reflection type color liquid crystal display high-contrast, an increase of the wavelength dispersion property of a polymer film used as a retardation film has become one of the technical subject matters.
Further, although these films are excellent in transparency, the thermal resistance and the mechanical properties (for example, rigidity) thereof are not always enough. Therefore, irregularity in dimensional or optical property is liable to occur, and in particular, by an environmental variation at the time of processing or using, they are likely to change in dimensional or optical property. Furthermore, if a raise of the processing temperature or a further thinning is required in the future, there is a limit for such an application in the above-described conventional films.
On the other hand, although a polyimide is well known as a polymer having a thermal resistance, because a usual polyimide is colored with a brown color, it cannot be applied for an optical use. As a polyimide having a transparency, for example, a wavelength plate is disclosed in Japanese Patent No. 3259563. However, because the polyimide according to this invention takes two hours or more for thermal imidization reaction, it is difficult to use this polyimide industrially. Further, this patent document does not touch Young's modulus which is an important factor for an optical film, at all.
Further, although aromatic polyamide films have high thermal resistance and mechanical properties and they can satisfy the thermal resistance and mechanical properties required for retardation films or protection films sufficiently, a para-oriented aromatic polyamide such as PPTA is colored with a yellow color, and it was difficult to develop such a polyamide for an optical field. For example, although a thermal-resistance transparent conductive film is disclosed in JP-B-7-89452, the transmittance at a wavelength of 600 nm of this film is low to be 71% even in the examples, and because the transmittance at a lower wavelength side is further low, it is not practical. Further, although discloses an aromatic polyamide film having a specific structure, a film having a high transparency cannot be obtained from the reason that the molar fraction of the specific structure is low, etc.
It could be advantageous to provide polyamides and polyamide films having a high rigidity, a high thermal resistance and a high transparency, and optical members using the same and copolymers of the polyamides.